ui: Compile boost:signals2 only once

Summary:

ui is one of the modules that poison other modules with boost/signals2
headers. This moves the include to the cpp file and uses a forward
declaration in the header.

Backport of core PR13634.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5051